Cardiac pacing is delivered to patients to treat a wide variety of cardiac dysfunctions. Cardiac pacing is often delivered by an implantable medical device (IMD), which may also provide cardioversion or defibrillation in response to detected cardiac tachyarrhythmias, if needed. The IMD delivers such stimulation to the heart via electrodes located on one or more leads, which are typically intracardiac leads.
Patients with heart failure may be treated with cardiac resynchronization therapy (CRT). CRT is a form of cardiac pacing. The ventricles of some heart failure patients contract in an uncoordinated, or asynchronous, manner, which greatly reduces the pumping efficiency of the ventricles. CRT delivers pacing pulses at particular times, e.g., atrio-ventircular (A-V) intervals and/or intra-ventricular (V-V) intervals, and particular locations, e.g., to one or both of the right and left ventricles, to re-coordinate the contraction of the ventricles. In some examples, CRT involves delivery of pacing pulses to both ventricles to synchronize their contraction. In other examples, CRT involves delivery of pacing pulses to one ventricle, such as the left ventricle, to synchronize its contraction with that of the right.
The effectiveness of CRT in improving a patient's cardiac function, referred to as CRT response, may be negatively affected when a left-ventricular (LV) pacing electrode is located proximate scar (or otherwise abnormal) substrate, e.g., because the abnormal substrate may not depolarize, and therefore not effectively propagate the depolarization throughout the left ventricle, in response to the pacing stimulus. Abnormal substrate may include epicardial and/or transmural scar substrate, as well as other abnormal substrate, such as fibrosis. During implantation, an LV lead may be repositioned to avoid abnormal substrate. Some LV leads include multiple electrodes available for selection for delivery of LV pacing, and may be referred to as multipolar leads. During or after implantation, a different electrode of a multipolar LV lead may be selected to avoid pacing via an electrode located proximate abnormal LV substrate.
One type of LV mapping procedure involves advancing a closely-spaced bipolar electrode pair at a distal end of a mapping catheter to a variety of LV locations, and determining the amplitude of ventricular depolarizations within the bipolar electrogram sensed by the bipolar electrode pair at the various LV locations. Amplitudes below a threshold are considered indicative of scar substrate at the location of the bipolar pair. Typically, the electrogram is sensed during intrinsic conduction, e.g., with ventricular activation via the His-Purkinje system during sinus rhythm, and the threshold is valid under these circumstances. Such an LV mapping procedure may be performed prior to LV lead implantation, or for patients with hemodynamically intolerable ventricular tachycardia, to identify scar substrate.